1. Field of the Invention
This invention relates to ring-laser gyroscopes. More specifically, the invention is a ring-laser gyroscope system that incorporates one or more dispersive elements to improve sensitivity while suppressing the dead-band.
2. Description of the Related Art
Ring-laser gyroscopes are well known devices used in a variety of navigation, alignment, measurement and/or surveying systems. The operating principles of a ring-laser gyroscope will be explained briefly with the aid of FIG. 1 where an embodiment of a ring-laser gyroscope is illustrated and referenced generally by numeral 10. Three (or more) mirrors 12, 14 and 16 are positioned to define an optical path 18 that reflects off mirrors 12, 14 and 16 to thereby create a continuous and ring-shaped path. A laser light source 20 pumps counter-propagating, continuous-wave laser beams at a selected lasing frequency along path 18 as indicated by arrows 22 and 24. Then, when ring-laser gyroscope 10 is rotated about a central axis 26 as indicated by rotation direction arrow 28, a beat frequency is defined between counter-propagating laser beams 22 and 24. That is, laser beam 24 propagating in the same direction as rotation direction 28 undergoes a first frequency shift while laser beam 22 propagating in the direction opposite to that of rotation direction 28 undergoes a second frequency shift. The difference between the two frequency shifts is known as the beat frequency and is proportional to the rate of rotation. The beat frequency is detected by a beat-note detector 30.
Unfortunately, conventional ring-laser gyroscopes suffer from a dead-band in their response as a result of backscattering as the ring-laser gyroscope rotates. More specifically, at mirrors 12, 14 and 16, minute amounts of backscattered light (at backscattered frequencies) is generated as light from laser beams 22 and 24 interact. This effectively couples beams 22 and 24 together. Light at the backscattered frequencies reduces the gyroscope's sensitivity to low rates of rotation. The dead-band can be avoided by biasing the gyroscope via incorporation of a dithering mirror or by using the Faraday effect. However, a dithering mirror is a moving and noisy optical component while introduction of the Faraday effect causes the gyroscope to be sensitive to magnetic field and temperature variations. The dead-band can be completely eliminated by using short laser pulses. However, while a short-pulse-laser ring-laser gyroscope performs well, shot noise associated with the laser pulses introduces the need to incorporate complex active stabilization. In addition, short pulse lasers require a large pump laser that is typically large and heavy.